Demand pacer with heart rate memory

ABSTRACT

A demand controlled cardiac pacer including a pair of electrodes for connection with the heart, a variable frequency relaxation oscillator connected to the electrodes, a resetting circuit for disabling the oscillator when heart pulses are produced at a normal rate, and a circuit responsive to the rate of reset for modifying the frequency of the oscillator so that upon heart failure stimulating pulses will be applied to the electrodes at a rate that begins somewhat below the last rate of production of natural pulses and gradually decreases to a fixed minimum rate.

United States Patent Cole [ 51 Sept. 26, 1972 [54] DEMAND PACER WITHHEART RATE MEMORY [72] Inventor: Addison D. Cole, Natick, Mass.

[73] Assignee: Adcole Corporation, Waltham,

Mass.

[22] Filed: Nov. 2, 1970 [21] Appl. No.: 86,107

52 us. Cl ..12s/419 P 51 Int. Cl. ..A61n 1/36 58 Field of Search..128/2.06 A, 2.06 F, 2.06 R,

[56] References Cited UNITED STATES PATENTS 3,528,428 9/1970 Berkovits128/419 P 3,596,718 7/1971 Krasner et a1. ..128/419 P PrimaryExaminer-William E. Kamm Attorney-Rich & En'cson [5 7] ABSTRACT A demandcontrolled cardiac pacer including a pair of electrodes for connectionwith the heart, a variable frequency relaxation oscillator connected tothe electrodes, a resetting circuit for disabling the oscillator whenheart pulses are produced at a normal rate, and

a circuit responsive to the rate of reset for modifying the frequency ofthe oscillator so that upon heart failure stimulating pulses will beapplied to the electrodes at a rate that begins somewhat below the lastrate of production of natural pulses and gradually decreases to a fixedminimum rate.

1 1 Claims, 4 Drawing Figures PATENTED I972 v 3.693.626

sum 1 or 2 R3 gRs M/VE/VTOR U ADDISON D. COLE BY wawm/ ATTORNEYSPA'TENTED 1912 3.693; 626

SHEEI 2 0F 2 H- v I!!! H6 3 s |||||.1| llll I/Vl EIVTOI? ADDISON D. COLEATTORNEYS 1 DEMAND PACER WITH HEART RATE MEMORY My invention relates tocardiac pacers, and particularly to a novel cardiac pacer with which therate at which stimulating pulses is produced is determined by the userso long as natural pulses are at least intermittently produced. The termstimulating pulses as used herein is in accordance with well-knownterminology meaning that the stimulating pulse functions as a stimulusto evoke the characteristic physiologic activity of a nerve or muscle,more particularly as used herein, to the heart. The stimulating pulsemay be either naturally derived or artificially produced.

Implantable cardiac pacers have been developed for the purpose ofsupplying stimulating pulses to actuate the heart when the bodymechanism fails to provide such pulses. One such pacer comprises a pairof electrodes adapted to be connected in electrical circuit with themyocardium, and a fixed rate relaxation oscillator connected to theelectrodes to supply stimulating pulses to the heart at a rate lowerthan the normally ex- 'pected rate of natural R waves. The apparatusgenerally comprises means responsive to the signals appearing on theelectrodes for resetting the oscillator to inhibit its operation whennatural pulses occur at a normal rate. Such apparatus is reasonably wellsuited for bed patients whose heart rates may be expected to be more orless constant. However, for ambulatory patients, there is the problemthat the normal heart rate varies considerably depending upon the levelof activity of the patient, as well as on emotional and digestivefactors. Should an episode of heart block occur in a patient whose heartwas beaking rapidly, the transition to the relatively low rate necessarywith a fixedrate pacer could have unpleasant and possibly dangerousconsequences. The object of my invention is to improve the fidelity withwhich pacers supplement the action of the natural heart.

Briefly, the above and other objects of my invention are attained by anovel pacer in which a variable frequency, resettable relaxationoscillator is employed. Heart sensing and stimulating electrodes areprovided for connection to the body of the host. The output terminals ofthe oscillator are connected to these electrodes. A pulse generator isconnected to the electrodes to produce a control pulse each time anelectrical signal of sufficient magnitude appears across the electrodes.Preferably, a pulse rate measuring circuit is provided that iscontrolled by the pulse generator to produce a signal indicative of therate at which such pulses occur on the electrodes. The oscillator isreset by signals produced when both the pulse generator produces anoutput pulse and the pulse rate measuring circuit indicates that therate of pulse production is within a predetermined range. A timingcircuit is provided which measures the rate at which the oscillator isreset, and adjusts the frequency of the oscillator to correspond withthat rate. If natural heart pulses fail to occur over a relatively longperiod, the oscillator rate falls to a minimum rate that is selected tobe below the rate at which normal pulses would occur. The'pulse ratemeasuring circuit is provided to protect against interference from straysignals that may be coupled into the circuits. Thus, in the absence ofany pulses, either of noise or of natural physiological origin, theapparatus of my invention functions as a conventional demand pacer.However, in the presence of intermittent pulses or only occasionalstoppages, the pacer acts to supplement the natural heart at a ratedetermined by the patients needs.

The manner in which the apparatus of my invention is constructed, andits mode of operation, will best be understood in the light of thefollowing detailed description, together with the accompanying drawings,of a preferred embodiment thereof. In the drawings;

FIG. 1 is a schematic wiring diagram of a variable rate pacer inaccordance with my invention;

FIG. 2 comprises a composite graph of waveforms encountered in theoperation of my invention;

FIG. 3 is a composite graph illustrating other waveforms encountered inthe operation of the apparatus of my invention; and

FIG. 4 is a composite graph of waveforms illustrating still furtherfeatures of the operation of the apparatus of my invention.

Referring now to FIG. 1, Ihave shown a cardiac pacer incorporating apair of electrodes 1 and 2. One of the electrodes is to be connected tothe myocardium of the patient. The other may be connected to anyconvenient location elsewhere in the body to serve as a reference andreturn electrode. The electrode 2 has been shown at a reference groundpotential, for convenience of exposition, and not to imply anyparticular requirement of the circuit. The electrodes 1 and 2 serve todetect naturally occurring heart waves, and at times to applystimulating electrical pulses to the heart to substitute for missingnatural heart signals.

Referring to FIGS. 2a, naturally occurring heart waves have the generalwaveform shown, and are divided for purposes of description and analysisinto the P, Q, R, S and T components familiar to those skilled in theart. Of these, the R wave is normally of considerably higher amplitude,and is selected for purposes of marking the interval between beats andto time the application of stimulating substitute pulses.

Returning to FIG. 1, the electrodes 1 and 2 are connected to the inputterminals of a conventional amplifying and limiting network A1 which maycomprise conventional circuits for detecting the R waves selectively,and producing corresponding pulses H as indicated in FIG. 2b.

Referring again to FIG. 1, the pulse produced by the amplifier A1 servesto trigger a conventional one-shot multivibrator 0S and cause it toproduce an output pulse of predetermined duration. The pulse so producedis assumed to be a positive, rectangular pulse.

The active output terminal of the one-shot multivibrator OS is connectedto one input terminal of a conventional AND gate G that is arranged toproduce a positive output signal when and only when two positive signalsare applied to its input terminals. The output terminal of themultivibrator OS is also connected through a capacitor C1 to the base ofa conventional pnp transistor Q1 that serves as an electronic switch.

The collector of the transistor O1 is returned to ground through aresistor R1. The emitter of the transistor 01 is connected to groundthrough a capacitor C2, and through a resistor R2 to the positiveterminal of battery B.

The resistor R2 and the capacitor C2 comprise a timing circuit producinga maximum voltage across the capacitor C2 that is a function of the timebetween discharges of that capacitor. When the transistor Q1 is biasedinto conduction, the capacitor C2 is discharged through the transistorQ1 and the resistor R1. The resistor R1 is selected to be small relativeto the resistor R2; it is included merely to protect the transistor Q1against excessive currents. As indicated, the transistor Q1 is turned onat the trailing edge of each pulse produced by the multivibrator OS.

The voltage across the capacitor C2 is compared with a reference voltagedeveloped across a resistor R4. The resistor R4 is connected betweenground and one terminal of a resistor R3 that has its other terminalconnected to the positive terminal of the battery B.

The junction between the resistor R2 and the capacitor C2 is connectedto the non-inverting input terminal of a conventional operationalamplifier A2. The junction of the resistors R3 and R4 is connected tothe inverting input terminal of the amplifier A2. The amplifior A2 isprovided with a conventional feedback resistor R5.

The output terminal of the amplifier A2 is connected to the second inputterminal of the AND gate G. The amplifier A2 will produce a positiveoutput signal to enable the gate G when the voltage across the capacitorC2 has reached a predetermined value.

The output terminal of the gate G is connected to the base of an npntransistor Q2 that serves as an electronic switch. The emitter of thetransistor Q2 is connected to ground. The collector of the transistor Q2is. returned to the positive terminal of the battery B through atransistor R6. The collector of the transistor Q2 is returned to groundthrough capacitor C3 and a resistor R7 connected in series. The resistorR7 is selected to be small with respect to the resistor R6, and servesprimarily to protect the transistor Q2 against excessive currents whenit is biased into conduction to discharge capacitor C3. I

.The collector of the transistor Q2 is connected to the emitter of aunijunction transistor Q4 connected as a relaxation oscillator. The baseof the transistor Q4 that is more remote from the emitter, commonlytermed base one is returned to ground through a resistor R14. The otherbase, commonly termed base two, is connected to the positive terminal ofthe battery B through a resistor R13.

The minimum rate of oscillation of the relaxation oscillator comprisingthe transistor O4 is determined by the time constant of the resistor R6and the capacitor C3. The resistor R6 is connected in shunt with asecond circuit for modifying the period of the oscillator. That circuitextends from the emitter of the unijunction transistor Q4 through thecollector-to-emitter path of a pnp transistor Q3, and thence through aresistor R12 of the positive terminal of the battery B. When thetransistor 03 is cut off, the relaxation oscillator comprising theunijunction transistor Q4 oscillates at the rate set by the resistor R6and the capacitor C3. When the transistor C3 is biased into conduction,depending on the extent of the bias, more or less current flows throughthe resistor R12 and increases the frequency of oscillation.

A control circuit for adjusting the extent of conduction of thetransistor O3 is responsive to the rate at which the transistor O2 isturned on to discharge the capacitor C3. For that purpose, anon-inverting amplifier A3 has its active input terminal connected tothe collector of the transistor 02. The active output ter minal of theamplifier A3 is connected to a peak detecting circuit comprising diodeD1, and a capacitor C4 connected in parallel with a resistor R10 betweenthe cathode of the diode D1 and ground.

The cathode of the diode D1 is also connected to the non-inverting inputterminal of a conventional operational amplifier A4. The inverting inputterminal of the amplifier A4 is connected to the junction of a pair ofresistors R8 and R9. The other terminal of the resistor R9 is connectedto ground, and the other terminal of the resistor R8 is connected to thepositive terminal of the battery B. The amplifier A4 is provided with aconventional feedback resistor R1 1.

The active output terminal of the amplifier A4 is connected to the baseof the pnp transistor Q3. The positive potential across the resistor R9tends to produce a negative potential biasing the transistor Q3 intoconduction. A positive potential across the capacitor C4 tends to resistthis reference potential, and turn ofi the transistor Q3. Thus, if thecapacitor C4 is charged to a sufficiently high voltage, the transistorQ3 will be cut off.

The voltage across the capacitor C4 will depend upon the period betweendischarges of the capacitor C3. If it is discharged at the minimum rate;i.e., under conditions when the transistor Q3 remains cut off and theoscillator comprising the unijunction transistor Q4 oscillates at theminimum rate, the peak voltage reached across the capacitor C3 will berelatively high, and the voltage across the capacitor C4 will be highenough to keep the transistor Q3 turned ofi. On the other hand, if thecapacitor C3 is more frequently discharged, a lower voltage will bereached across the capacitor C4, and the transistor Q3 will conduct moreheavily, increasing the frequency of oscillation of the oscillatorcomprising the transistor Q4.

Base one of the transistor Q4 is connected to the base of a npntransistor Q5 that serves as an electronic switch. The emitter of thetransistor OS is connected to ground. The collector of the transistor Q5is returned to the positive terminal of the battery B through a resistorR15. The collector of the transistor O5 is connected to the stimulatingelectrode 1 through a capacitor C5. Thus, during the period between thetime when the unijunction transistor Q4 is fired, and the time that thecapacitor C3 has discharged sufliciently to cut off the transistor Q4,the collector of the transistor Q5 will go essentially to groundpotential and thereby apply a rectangular negative pulse to theelectrode 1.

The polarity of natural electrical waves produced in the myocardium withrespect to the ground connection provided elsewhere in the body willdetermine whether it is the electrode 1, connected to the collector ofthe transistor Q5, or the electrode 3, that is actually connected to themyocardium. As that matter of polarity will not trouble the myocardium.As that matter of polarity will not trouble the artisan, it will beignored in the following discussion, and in the graphs of FIG. 2 through4, where all pulses have been indicated as positive to simplify thediscussion.

The apparatus of P161 is preferably enclosed in a suitable biologicallycompatible container for implantation. As will be apparent to thoseskilled in the art, the circuits of FIG. 1 are well adapted toconstruction by miniature circuit techniques familiar to those skilledin the art. Thus, the pacer need occupy but little space.

Having thus described the construction of the apparatus of my invention,its operation will next be described in connection with the diagrams ofFIG. 2 through 4. As noted above in connection with FIG. 2, the pulses Hare produced by the amplifier A1 in response to each naturally occurringR wave. Fig. 3a shows a series of such pulses H, followed by a heartblock episode in which no pulses are produced. As illustrated in FIG.3b, when a pulse H fails to occur, a pulse S is produced a short timeafter it should have occurred. Pulses S continue to be produced at arate that is initially slightly below the rate at which H pulses hadbeen produced. That rate gradually tapers to a minimum rate below therate at which natural pulses should be produced. When heart beats areresumed, the pulses S will cease.

The details of the manner in which the apparatus in FIG. 1 functions toproduce the operation illustrated in FIG. 3, together with its mode ofoperation in the presence of noise, will next be described in connectionwith FIG. 4.

Referring to FIG. 40, I have shown two initial naturally produced pulsesH that are assumed to beat the end of a train of such pulses occurringover a sufficiently long interval that the operation of the apparatus inFIG. 1 has become stabilized. Prior to the first of the pulses H shownin FIG. 4, the capacitor C2 has been charging, as illustrated in FIG.4b. When the first pulse H occurs, the one-shot multivibrator OS istriggered to produce a pulse as illustrated in FIG. 4d.

It is assumed that the capacitor C2 has charged to a value that willovercomethe reference voltage appearing across the resistor R4, andthereby enable the gate G to produce a positive signal in response tothe pulse from the one-shot multivibrator OS. That pulse will turn onthe transistor Q2 and discharge the capacitor C3 through the resistorR7. The capacitor C3 will remain discharged during the pulse produced bythe multivibrator OS. At the trailing edge of that pulse, the capacitorC2 will be discharged, as shown in FIG. 4b. That capacitor will begin tocharge again, and the cycle will be repeated when the next pulse H isproduced.

Next, it is assumed that a heart block episode occurs in which an R wavedoes not occur within the current period of oscillation of themultivibrator comprising the unijunction transistor Q3. The capacitor C2will continue to charge, and the capacitor C3 will be charged until itreaches the unijunction firing potential, shown as Vt in FIG. 4c.

I During the charging of the capacitor C3, the time constant isdetermined both by the resistor R6 and by the resistor R12 in serieswith the conducting transistor 03. The discharge of the capacitor C3thus occurs more rapidly than it would if the transistor Q3 was cut off,and a stimulating pulse S is produced after an interval longer than butclose to the interval between previously appearing pulses H. When thepulse thus is produced, it will retrigger the one-shot multivibrator 08.At the trailing edge of the multivibrator pulse, the capacitor C2 willbe discharged.

Since the interval between the last pulse H and the stimulating pulse Swas longer than the interval between the previous pulses H, the peakdetecting circuit, comprising the diode DI, the capacitor C4 and theresistor R10, will apply a higher voltage to the amplifier A4, reducingthe current in the emitter-to-collector path of the transistor 03, andthereby lowering the rate at which the capacitor C3 will charge.Accordingly, assuming no intervening pulse H, the next pulse S producedby the circuit in FIG. 1 will follow the previous pulse S by a somewhatlonger interval than that pulse followed the last H pulse. If succeedingpulses S continue to be produced in the absence of pulses H, theinterval would gradually increase until the transistor 03 was completelyout off, whereupon the pulses 8 would continue to be produced at a fixedrate.

FIG. 4a illustrates a noise pulse N occurring shortly after the secondpulse S. That pulse will trigger the oneshot multivibrator OS and causethe capacitor C2 to be discharged before it reaches the voltage Vg atwhich the gate G could be enabled. Accordingly, the transistor Q2 willremain cut off the capacitor C3 will continue to charge until it reachesthe unijunction firing potential Vt. Thus, the oscillator comprising theunijunction transistor Q3 will continue to produce pulses at a ratedeclining towards the minimum rate so long as either natural pulses H ornoise pulses N are produced at a rate greater than the maximum expectedrate for natural pulses.

I While I have described my invention with respect to the details of thepreferred embodiment thereof, many changes and variations will occur tothose skilled in the art upon reading my description, and such canobviously be made without departing from the scope of my invention.

Having thus described my invention, what I claim is;

1. A variable rate demand pacer, comprising:

a pair of electrodes adapted to be connected in electrical circuit withliving tissue;

pulse generating means connected to said electrodes for producing afirst pulse each time an electrical signal above a predeterminedmagnitude appears on said electrodes;

pulse rate measuring means controlled by said pulse generating means forproducing an output signal when said first pulses are produced at a ratebelow a predetermined maximum rate;

a relaxation oscillator, comprising:

output terminals connected to said electrodes,

a timing circuit including a variable impedance,

a source of voltage, and

a capacitor connected in series, and

means responsive to the voltage across said capacitor for dischargingthe capacitor and producing a pulse across said output terminals whenthe voltage across the capacitor reaches a predetermined value;

an electronic switch connected in parallel with said capacitor;

gate means controlled by said pulse generating means and said pulse ratemeasuring means for closing said switch when said first pulse and saidoutput signal are both produced;

voltage detecting means connected to said capacitor to produce a controlsignal in accordance with the peak voltage developed across saidcapacitor; and

means responsive to said control signal for adjusting said variableimpedance to adjust the period of said oscillator to a rateapproximating the rate at which said electronic switch is closed.

2. In combination with a pair of sensing and stimulating electrodesadapted to be connected in electrical circuit with living tissue,

a resettable variable frequency oscillator comprising a resistor, acapacitor and a source of voltage connected in series;

a voltage responsive electron discharge device connected to saidcapacitor and responsive to a predetermined voltage across the capacitorto discharge it; a

said resistor and capacitor having a time constant selected to actuatesaid discharge device at intervals greater than the intervals 'betweennormal heartbeats;

means connected to said discharge device and responsive to discharge ofsaid capacitor therethrough to apply a pulse to said electrodes;

a variable impedance connected in parallel with said resistor;

means responsive to the average voltage across said capacitor foradjusting said impedance in accordance with said average voltage;

an electronic switch connected across said capacitor and effective whenclosed to discharge said capacitor independently of the voltage acrossit; and

means responsive to a pulse across said electrodes for closing saidswitch.

3. The apparatus of claim 2, in which said variable impedance comprises:

a second resistor, and a transistor having an emitter and a collectorconnected in series with said second resistor, and a base;

and in which said average voltage responsive means comprises peakdetecting means connected between said capacitor and said base forcontrolling the impedance between said emitter and said collector.

4. A demand pacer, comprising:

a pair of electrodes adapted to be connected in electrical circuit witha heart,

amplifyingvmeans connected to said electrodes to produce an outputsignal in response to a voltage on said electrodes having an amplitudecharacteristic of an R wave,

a pulse generator connected to said amplifying means to produce a pulsein response to each signal,

a first timing circuit comprising a first resistor, a battery, and afirst capacitor. connected in series;

a first electronic switch connected across said first capacitor todischarge it when said first switch is closed;

means responsive to the trailing edge of each pulse for closing saidswitch to discharge said first capacitor;

voltage sensing means controlled by said capacitor for Producing a firstcontrol signal when the volt age across said capacitor reaches apredetermined value;

gate means responsive to said pulses and said first control signals toproduce a second control signal when said first control signal occursduring a pulse;

of said capacitor to apply a pulse to said electrodes of sufficientamplitude to cause said amplifying means to produce an output signal.

5. A variable rate demand pacer, comprising: a pair of electrodesadapted to be connected in electrical circuit with living tissue;

pulse generating means connected to said electrodes for producing apulse each time a signal above a predetermined amplitude appears on saidelectrodes;

a resettable relaxation oscillator having an adjustable period connectedto said electrodes; means controlled by said pulse generating means andresponsive to said pulses to reset said oscillator; means responsive tothe rate at which said pulses are produced for adjusting the period ofsaid oscillator;

said oscillator comprising:

a source of voltage, a resistor, and a capacitor connected in series;

an electron discharge device connected across said capacitor andresponsive to a predetermined voltage across the capacitor to dischargethe capacitor;

an electronic switch connected across said capacitor and effective whenclosed to discharge the capacitor and thereby reset the oscillator; and

a variable impedance connected in parallel with said resistor; and

in which said rate responsive means comprises peak detecting meansresponsive to the peak voltage reached by said capacitor prior todischarge for producing a control signal, and means responsive to saidcontrol signal for adjusting said variable impedance.

6. A cardiac pacer normally responsive to natural heart stimulatingpulses in its operation and adapted to provide artificial heartstimulating pulses in the event of the absence of a natural pulse,comprising:

means for receiving natural heart stimulating pulses;

means for generating an artificial heart stimulating pulse; meansresponsive to said stimulating pulses for controlling said artificialpulse generating means to produce an artificial pulse only after avariable time interval, after the last said stimulating pulse;

means responsive to a plurality of previously naturally occurring pulsesfor defining said variable time interval as a function of said pluralityof previously occurring natural pulses; and

means for coupling artificial stimulating pulses to a natural heart.

7. The The pacer of claim 6, wherein:

said previously naturally occurring pulses responsive means includesmeans for storing a selected number of the last received natural pulsesand producing a signal which is a function of the average intervalbetween said selected pulses.

8. The pacer of claim 6, wherein:

said artificial pulse generating means tends to provide artificialpulses in sequence and said means for controlling said artificial pulsegenerating means includes means inhibiting said artificial pulsegenerating means from producing an artificial pulse during said variabletime interval after a received natural pulse. 1

9. The pacer of claim 6, wherein:

said artificial pulse generating means includes a resettable, variablerate, pulse generator.

10. The pacer of claim 7, wherein:

said artificial pulse generator is operative selectively to supplyartificial pulses in sequence at controllable intervals, saidcontrollable intervals, at least in itially, being determined as afunction of the most recent of a predetermined sequence of natural heartstimulating pulses.

l l. The pacer of claim 6, wherein:

said artificial pulse generator includes a resettable variable frequencyrelaxation oscillator, comprismg:

a source of voltage, a variable impedance and a capacitor connected inseries;

an electronic switch connected in parallel with said capacitor andeffective to discharge said capacitor when closed;

means responsive to the rate of discharge of said capacitor foradjusting said variable impedance{ and a voltage responsive electrondischarge device connected in parallel with said capacitor andresponsive to a predetermined voltage across the capacitor to dischargeit.

1. A variable rate demand pacer, comprising: a pair of electrodesadapted to be connected in electrical circuit with living tissue; pulsegenerating means connected to said electrodes for producing a firstpulse each time an electrical signal above a predetermined magnitudeappears on said electrodes; pulse rate measuring means controlled bysaid pulse generating means for producing an output signal when saidfirst pulses are produced at a rate below a predetermined maximum rate;a relaxation oscillator, comprising: output terminals connected to saidelectrodes, a timing circuit including a variable impedance, a source ofvoltage, and a capacitor connected in series, and means responsive tothe voltage across said capacitor for discharging the capacitor andproducing a pulse across said output terminals when the voltage acrossthe capacitor reaches a predetermined value; an electronic switchconnected in parallel with said capacitor; gate means controlled by saidpulse generating means and said pulse rate measuring means for closingsaid switch when said first pulse and said output signal are bothproduced; voltage detecting means connected to said capacitor to producea control signal in accordance with the peak voltage developed acrosssaid capacitor; and means responsive to said control signal foradjusting said variable impedance to adjust the period of saidoscillator to a rate approximating the rate at which said electronicswitch is closed.
 2. In combination with a pair of sensing andstimulating electrodes adapted to be connected in electrical circuitwith living tissue, a resettable variable frequency oscillatorcomprising a resistor, a capacitor and a source of voltage connected inseries; a voltage responsive electron discharge device connected to saidcapacitor and responsive to a predetermined voltage across the capacitorto discharge it; said resistor and capacitor having a time constantselected to actuate said discharge device at intervals greater than theintervals between normal heartbeats; means connected to said dischargedevice and responsive to discharge of said capacitor therethrough toapply a pulse to said electrodes; a variable impedance connected inparallel with said resistor; means responsive to the average voltageacross said capacitor for adjusting said impedance in accordance withsaid average voltage; an electronic switch connected across saidcapacitor and effective when closed to discharge said capacitorindependently of the voltage across it; and means responsive to a pulseacross said electrodes for closing said switch.
 3. The apparatus ofclaim 2, in which said variable impedance comprises: a second resistor,and a transistor having an emitter and a collector connected in serieswith said second resistor, and a base; and in which said average voltageresponsive means comprises peak detecting means connected between saidcapacitor and said base for controlling the impedance between saidemitter and said collector.
 4. A demand pacer, comprising: a pair ofelectrodes adapted to be connected in electrical circuit with a heart,amplifying means connected to said electrodes to produce an outputsignal in response to a voltage on said electrodes having an amplitudecharacteristic of an R wave, a pulse generator connected to saidamplifying means to produce a pulse in response to each signal, a firsttiming circuit comprising a first resistor, a battery, and a firstcapacitor connected in series; a first electronic switch connectedacross said first capacitor to discharge it when said first switch isclosed; means responsive to the trailing edge of each pulse for closingsaid switch to discharge said first capacitor; voltage sensing meanscontrolled by said capacitor for producing a first control signal whenthe voltage across said capacitor reaches a predetermined value; gatemeans responsive to said pulses and said first control signals toproduce a second control signal when said first control signal occursduring a pulse; a second electronic switch controlled by said gate meansand closed by said second control signal; a second capacitor connectedacross said switch; a second resistor and a source of voltage connectedin series with said capacitor; an electrically variable impedanceconnected in parallel with said second resistor; means responsive to thepeak voltage developed across said second capacitor for adjusting saidvariable impedance; a voltage responsive electronic discharge deviceconnected across said capacitor to discharge it in response to apredetermined voltage across the capacitor, and means responsive to thedischarge of said capacitor to apply a pulse to said electrodes ofsufficient amplitude to cause said amplifying means to produce an outputsignal.
 5. A variable rate demand pacer, comprising: a pair ofelectrodes adapted to be connected in electrical circuit with livingtissue; pulse generating means connected to said electrodes forproducing a pulse each time a signal above a predetermined amplitudeappears on said electrodes; a resettable relaxation oscillator having anadjustable period connected to said electrodes; means controlled by saidpulse generating means and responsive to said pulses to reset saidoscillator; means responsive to the rate at which said pulses areproduced for adjusting the period of said oscillator; said oscillatorcomprising: a source of voltage, a resistor, and a capacitor connectedin series; an electron discharge device connected across said capacitorand responsive to a predetermined voltage across the capacitor todischarge the capacitor; an electronic switch connected across saidcapacitor and effective when closed to discharge the capacitor andthereby reset the oscillator; and a variable impedance connected inparallel with said resistor; and in which said rate responsive meanscomprises peak detecting means responsive to the peak voltage reached bysaid capacitor prior to discharge for producing a control signal, andmeans responsive to said control signal for adjusting said variableimpedance.
 6. A cardiac pacer normally responsive to natural heartstimulating pulses in its operation and adapted to provide artificialheart stimulating pulses in the event of the absence of a natural pulse,comprising: means for receiving natural heart stimulating pulses; meansfor generating an artificial heart stimulating pulse; means responsiveto said stimulating pulses for controlling said artificial pulsegenerating means to produce an artificial pulse only after a variabletime interval, after the last said stimulating pulse; means responsiveto a plurality of previously naturally occurring pulses for definingsaid variable time interval as a function of said plurality ofpreviously occurring natural pulses; and means for coupling artificialstimulating pulses to a natural heart.
 7. The The pacer of claim 6,wherein: said previously naturally occurring pulses responsive meansincludes means for storing a selected number of the last receivednatural pulses and producing a signal which is a function of the averageinterval between said selected pulses.
 8. The pacer of claim 6, wherein:said artificial pulse generating means tends to provide artificialpulses in sequence and said means for controlling said artificial pulsegenerating means includes means inhibiting said artificial pulsegenerating means from producing an artificial pulse during said variabletime interval after a received natural pulse.
 9. The pacer of claim 6,wherein: said artificial pulse generating means includes a reSettable,variable rate, pulse generator.
 10. The pacer of claim 7, wherein: saidartificial pulse generator is operative selectively to supply artificialpulses in sequence at controllable intervals, said controllableintervals, at least initially, being determined as a function of themost recent of a predetermined sequence of natural heart stimulatingpulses.
 11. The pacer of claim 6, wherein: said artificial pulsegenerator includes a resettable variable frequency relaxationoscillator, comprising: a source of voltage, a variable impedance and acapacitor connected in series; an electronic switch connected inparallel with said capacitor and effective to discharge said capacitorwhen closed; means responsive to the rate of discharge of said capacitorfor adjusting said variable impedance; and a voltage responsive electrondischarge device connected in parallel with said capacitor andresponsive to a predetermined voltage across the capacitor to dischargeit.